An integrated circuit is a sensitive device, which may be damaged by high peak currents resulting from external overshooting events. External overshooting events on an integrated circuit may result from an external source discharging a large transient voltage over a short period of time onto a terminal of the integrated circuit, causing the operating voltage to suddenly increase. Such events may include, for example, electrostatic discharge events resulting from human body contact with the integrated circuit or power up glitches.
Damage may be prevented by coupling a protection circuit to the integrated circuit, the protection circuit being activated when the operating voltage of the integrated circuit exceeds the trigger voltage—or activation voltage—of the protection circuit. The trigger voltage of the protection circuit should be just above the highest breakdown voltage of the transistors on the integrated circuit. This ensures that the protection circuit is inactive, or non-conducting, during normal integrated circuit operation, but activates at the lowest possible voltage beyond the operational limits of the integrated circuit transistors. When the trigger voltage of the protection circuit is exceeded, it will turn on and conduct current until the excessive voltage from the integrated circuit drops below the holding voltage of the protection circuit—i.e., the voltage below which the protection circuit no longer passes current.
The silicon controlled rectifier is a protection circuit recognized as an effective device for on-chip protection against external overshooting events. In its “on,” or conducting, state, the silicon controlled rectifier provides excellent protection for sensitive devices because its “on” resistance and holding voltage may be very low. To activate the silicon controlled rectifier, sufficient voltage must be applied to avalanche a p-n junction, thus allowing the silicon controlled rectifier to pass current.
The silicon controlled rectifier may suffer, however, from certain problems. For example, the silicon controlled rectifier may experience latchup and be destroyed. Latchup results when a circuit draws a large current while maintaining a low voltage across its terminals—e.g., a short circuit. To avoid latchup, it may be desirable to increase the holding voltage of the silicon controlled rectifier, thus turning off the silicon controlled rectifier, which receives voltage from a Vdd voltage source, and preventing damage that would otherwise result. Further, because the trigger current of the silicon controlled rectifier is generally low, the silicon controlled rectifier may be accidentally triggered on by external overshooting or undershooting noise on the integrated circuit in normal operation.
It is desirable for a protection circuit to draw negligible leakage current during its inactive state, and to allow the integrated circuit to operate without interruption or malfunction. It is also desirable for a protection circuit to quickly activate and pass current when the voltage tolerance of the integrated circuit is exceeded, as occurs during external overshooting events. It is further desirable for a protection circuit in its active state to have a low impedance and a low holding voltage, thus dissipating very high external overshooting/electrostatic discharge peak currents and voltages without damaging either the integrated circuit or the protection circuit.
Previous solutions to external overshooting and electrostatic discharge events have experienced various drawbacks. For example, U.S. Pat. No. 5,012,317 discloses a silicon controlled rectifier that may be turned on by avalanching an intermediate p-n junction, which places the device in a current passing mode (“regenerative mode”) for positive transients. However, negative transients, which may forward bias the p-n junction, may interrupt the integrated circuit's operation or cause the integrated circuit to malfunction because of substrate bouncing.
As another example, U.S. Pat. No. 5,212,618 discloses a protection circuit including a pair of transistors, which protect an integrated circuit from peak voltages. However, the protection circuit has a high holding voltage, which is potentially damaging to the integrated circuit as the power dissipation during activation will be high. In addition, the circuit offers no protection when an electrostatic discharge event occurs between two circuit terminals, as the circuit only allows for connection between a circuit terminal and the substrate.
As yet another example, U.S. Pat. No. 5,392,185 discloses a silicon controlled rectifier; however, it may only be applied in GaAs material. It may not be applied in a silicon integrated circuit process.
A protection circuit is accordingly desired in which the trigger voltage and holding voltage can be adjusted, which prevents accidental triggering of the silicon controlled rectifier, while still protecting the integrated circuit from high peak currents, and which obtains latchup immunity for systems with various Vdd voltage supplies.